This invention relates to a thermal printing head, and more particularly to a conductor pattern formed on a ceramic substrate for a thermal printing head.
A heat-resisting glass layer is formed on a ceramic substrate, and on this glass layer, a line of heat generating resistor is formed. A line of power supply common conductor is formed in parallel to the line of the heat generating resistor. Equally spaced power-supply-side conductors are formed originating from the power supply common conductor and intersecting the heat generating resistor in perpendicular direction for making contacts to the heat generating resistor.
Each ground-side conductor is formed intersecting the heat generating resistor at a middle point adjacent two power-supply-side conductors.
This invention relates to the pattern of these conductors, and the background of the invention will be explained in connection with FIG. 5, which shows the circuit diagram of a thermal printing head. In FIG. 5, 1 is heat generating resistor, 2 is shift register, 3 is latch, 4 is driver, 5 is power supply, 60 is power supply common conductor, 7 is power-supply-side conductor, 8 is ground-side conductor. Control circuits 10 are composed of integrated circuits including the shift register 2, the latch 3, and the driver 4.
In accordance with the dot pattern of a line to be printed, a bit pattern in the shift register 2 is arranged. This bit pattern is latched by the latch 3. Each unit of the driver 4 connects the corresponding ground-side conductor 8 to ground in accordance with the logic of the corresponding bit on the latch 3 during the time interval of a strobe signal from control lines.
For example, when a ground-side conductor 8a is grounded through the control circuits 10, current flows form the power-supply-side conductors 7a, 7b to the ground-side conductor 8a, and a section indicated by 1a of the heat generating resistor 1 is heated.
The operator of the thermal printing equipment naturally wishes to see a part of printing immediately after the printing of the part is finished. In a so-called edge-type thermal printing head, the heat generating resistor 1 is formed near to an edge line of the ceramic substrate so that the printed portion quickly leaves the edge line of the thermal printing head to an open space visible by the operator.
When the heat generating resistor 1 is near to an edge line of the ceramic substrate, only a narrow space is left for the power supply common conductor 60 between the heat generating resistor 1 and the edge line of the substrate.
Narrower width of the power supply common conductor 60 means larger resistance. In heretofore known thermal printing heads, the power supply common conductor 60 is connected to the power supply only at both ends of the conductor 60. Voltages impressed to heat generating sections in the central parts of the heat generating resistor 1 become lower than those impressed to heat generating sections near to both ends because of the voltage drop in the power supply common conductor 60. This difference of impressed voltages deteriorates the quality of printing.
In heretofore known thermal printing heads, there are no means for compensating for the voltage drop in the power supply common conductor 60.